Storage constrained synchronization of shared content items

ABSTRACT

A content management system synchronizes content items across client computing systems connected by a network. Each client device has a storage allocation for synchronized shared content items. If the storage allocation for shared content items on a client device is exceeded by the request to add or edit a content item such that it is enlarged, or open a large content item remote to the client device, a client application or the host of content management system selects content items to remove from residence on the client device but keep remotely on content management system. Upon removal of the selected content items, the client application creates shadow items, representing the content item but only containing the metadata of the content item. This creates sufficient space for the initial request to be completed while maintaining user access to all synchronized shared content items.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/133,185, filed Apr. 19, 2016, now U.S. patent Ser. No. ______, whichis a continuation of U.S. application Ser. No. 14/611,115, filed Jan.30, 2015, now U.S. Pat. No. 9,361,349, each of which is incorporated byreference in its entirety.

BACKGROUND

The described embodiments relate generally to improving the performanceof computer systems providing content item synchronization, andparticularly to improving the synchronization of content items between aclient device and a content management system where storage allocationfor synchronized content items is constrained.

Content management systems enable users to share content items from oneclient to another client. The clients are computing devices that providecontent items to a content management system for storage andsynchronization with other clients. The other clients may be operated byanother user or may be devices registered or managed by the same user. Auser designates which content items or directories containing contentitems are available to be shared with other users, and thus synchronizedto the client devices of such users. Generally, a content managementsystem synchronizes a given content item with all of the client devicesthat have been designated to share the content item. As a result, eachof these client devices may store a very large amount of shared contentitems. In some cases, the amount of storage taken up on a client deviceby the shared content items substantially reduces the amount of storageavailable on the client device for other items, such as unsynchronizedcontent items and applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are concept diagrams that illustrate one embodiment ofconstrained synchronization.

FIG. 2 illustrates a system environment for a content management systemthat synchronizes content items between client devices.

FIG. 3 illustrates the software architecture of a client device.

FIG. 4 is an interaction diagram of constrained synchronization foraccessing an existing content item in the local content directory.

FIG. 5 is an interaction diagram of constrained synchronization forcreating a new content item to be stored in the local content directory.

FIG. 6 illustrates a system environment for host based constrainedsynchronization.

FIG. 7 is an interaction diagram of constrained synchronization managedby a host device.

FIG. 8 is an illustration of a user interface for a local contentdirectory with icons representing remote and local content items.

FIG. 9 is a concept diagram illustrating constrained synchronizationusing predicted content item importance.

FIG. 10 illustrates a system environment for a content management systemusing predicted content item importance for constrained synchronization.

FIG. 11 illustrates the software architecture of a client device usingidle state triggered content management.

FIG. 12 illustrates a system environment for a content management systemusing idle state triggered content management.

FIG. 13 is a flow diagram illustrating the process used in idle statetriggered content management.

The figures depict various embodiments for purposes of illustrationonly. One skilled in the art will readily recognize from the followingdiscussion that alternative embodiments of the structures and methodsillustrated herein may be employed without departing from the principlesof the invention described herein.

DETAILED DESCRIPTION Functional Overview of Constrained Synchronization

A general functional overview of a constrained synchronization systemand process is now described. As a preliminary condition, users storecontent items on client devices, and the content items are synchronizedwith instances of the content items on other clients and with a hostsystem, typically a content management system. A client device storesthe content items in a local content directory. Content items stored inthe local content directory are synchronized with a content managementsystem, which maintains copies of the content items and synchronizes thecontent items with other client devices. Each client device executes aclient application, which enables the user to access the contentmanagement system. The client application further enables the user toconfigure a maximum storage allocation or size for the local contentdirectory.

In one aspect, the client device is configured to selectively determinewhich synchronized content items remain locally available on the clientdevice, and which are stored in their entirety only on the contentmanagement system. In one embodiment, the client device receives arequest to access a content item, for example from an applicationneeding access to the content item. The client device determines whetherthe requested content item is a shadow item or a content item storedlocally on the client device. A shadow item is an item that representsor emulates the content item, but does not contain the application dataof the content item. Generally, the shadow item replicates the metadataattributes of the content item, such as the name of the content item, aswell as various attributes, such as type, path information, accessprivileges, modification information, and size of the content item,without storing the actual application content, such as text, imagedata, video data, audio data, database tables, spreadsheet data, graphicdata, source or object code, or other types of content data. Because theshadow items only store metadata for the content item, they require onlya small amount of storage, e.g., typically about four kilobytes, ascompared to a content item that can be hundreds of megabytes or evenseveral gigabytes in size. Thus, using shadow items to represent contentitems operates to save considerable storage space, thereby improving thefunctioning of the client device.

Where the client device determines that the requested content item is ashadow item, this indicates that the requested content item content isnot at present stored on the client device, but is stored on the contentmanagement system. Accordingly, the client device downloads from thecontent management system the content item that corresponds to therequested shadow item. The client device further determines whetherstoring the content item in the local content directory would exceed themaximum storage size established for that directory. In that case, theclient device determines which content item or items in the localcontent directory can be removed from the local content directory, andreplaced with shadow items that represent the content items. Generally,the client device uses one or more attributes of a shared content itemto select content items from the local content directory that have beendetermined as being unattended by the user of the client device or userswith access to the content item via the content management system,including latest access time on the client device (e.g., actions of theuser of the client device or applications executing thereon), latestaccess time on the other client devices with which the content items areshared (e.g., actions of the users of those client devices), contentitem size, and access frequency. Combinations of these factors may alsobe used to determine unattended content items. The client device selectsa number of content items from the local content directory such thatdeleting these content items creates a sufficient amount of storagespace in the local content directory to allow the downloaded contentitem to be stored therein without exceeding the maximum storage size. Inone embodiment, the client device selects a number of content items sothat the total amount of storage used by these content items in theshared content directory at least equals or exceeds the amount ofstorage required to store the downloaded content item.

The client device deletes the selected content items, and for eachdeleted content item creates a corresponding shadow item. The clientdevice stores the shadow items in the directory locations correspondingto the deleted content items. Storage of the shadow items in thecorresponding locations enables subsequent retrieval of the deletedcontent items in a manner that is transparent to the requestingapplications.

This embodiment provides a constrained shared storage system whereineach client device can maintain access to all content items shared witha content management system while having more space for other contentitems and applications, and improves the storage efficiency of eachclient device as well as content management system as a whole. Moreparticularly, the embodiment enables a client device to effectivelyoperate as if it has a significantly larger storage capacity than itdoes in fact. For example, a client device with only a 10 GB storageallocation for the local content directory can operate as if it had over4,000 GB (4 TB) of storage allocation for that directory, representing a400-fold increase in effective storage. In the past, such a solution tolimited local storage capacity was made impossible by networkconnectivity and bandwidth limitations, thus the problem being solvedfor arises as a result of the recent developments in Internetinfrastructure that allows for pervasive connectivity and fast uploadand download speeds.

Despite the recent developments in Internet infrastructure, thecomputational, uploading, and downloading times required for the removalof content items, their replacement with shadow items, and theirrestoration following a user request may still impact deviceperformance. Therefore, alternative embodiments are also described thatreduce impact on device performance as visible to the user while stillreducing the storage burden on a client device over traditional sharedcontent synchronization methods. In one embodiment, the computation,uploading, and downloading are completed based upon a predicted a useraccess to a shared content item represented as a shadow item. To predicta user access to a content item, the client application or the contentmanagement system maintains a retention score for each content item; theretention score is a measure of the predicted importance to the user ofeach content item. Each client device is configured with a retentionscore threshold such that any content item with a sufficiently highpredicted importance (represented by a retention score that exceeds aretention score threshold) is downloaded to the corresponding clientdevice. The retention score may be calculated based on a variety ofattributes including latest access time, location, type, size, accessfrequency, shared status, number of accounts with access, number ofdevices with access, or number of devices storing the content item.

Alternatively, another embodiment allows the storage space occupied byshared content items on a client device to exceed the storage allocationwhile the activity of the client device is monitored (either by thecontent management system or by the client application). When a clientdevice is determined as being idle, the client application removes thecontent items and replaces them with shadow items, as previouslydiscussed, in order to reduce the effective storage space occupied bythe content items stored on the client device. In these embodiments, thestorage allocation is not maintained at all times and so occupiedstorage can be reduced according to other content item attributes.Instead of maintaining a storage allocation, for example, all contentitems with a latest access date older than a specified amount of time(e.g., two weeks) could be removed and replaced with shadow itemswhenever the client device is idle. This process does not keep theoccupied storage space below a storage allocation but would reduce it ina way that might be preferable to the user since the operations are donewhile the client device is idle and thus not being actively used by theuser, thereby improving a client device configured to use a constrainedsynchronization system by offering a user experience improvement overthe previously described embodiments while providing a similar increasein effective storage capacity.

FIGS. 1A and 1B are concept diagrams that further illustrate embodimentsof constrained synchronization. FIG. 1A illustrates a process of savinga content item in a storage constrained synchronized folder. FIG. 1Billustrates a process of opening a shadow item on a storage constrainedclient device.

In FIGS. 1A and 1B, client device 100A is one of a plurality of usercontrolled devices that can be connected and synchronized with contentmanagement system 110. Content management system 110 is a serverinstantiated to synchronize content from a plurality of client devicesusing a network. A shared content storage directory 120 is a directorylocated on the client device 100 that contains content synchronized withcontent management system 110. A storage allocation 130 is a parametervalue that specifies an amount of storage space allowed for all contentitems in the shared content storage directory 120. The storageallocation 130 can be set by the user of the client device 100A, theoperating system of the client device 100, a client application ofcontent management system 110, by a system administrator, or by policiesestablished at content management system 110. An example value for thestorage allocation 130 is 10 GB; this means that the user can store upto 10 GB of content items in their entirety (all content item attributesand data) in the shared content storage directory 120. Content items 140are saved within the shared content storage directory 120; aftersynchronization between a client device 100 and content managementsystem 110 a version of each content item 140 in the shared contentstorage directory 120 is also maintained by content management system110.

The term “content item”, as used herein indicates any file, group offiles, or collection of files. Any content item that consists of only asingle file may alternatively be referred to as a file. Additionally,terms such as “file table” may be used to refer to both individual filesor content items.

In FIG. 1 the shared content storage directory 120 is graphicallydepicted as a box that contains the content items 140. The storageallocation 130 is represented by the particular length of the boxrepresenting the content storage directory 120.

The first illustration of the client device 100A and content managementsystem 110 represents a typical state of the two entities. The clientdevice has content items 140A, 140B, and 140C stored within its sharedcontent storage directory 120 (only a small number of content items 140are shown for the purpose of explanation, as in practice the number ofcontent items 140 can be in the thousands, tens of thousands, or more).Content management system 110 is represented as being synchronized withclient device 100A and so it maintains an identical version of each ofthe content items stored on the client device 100A though it does nothave a storage allocation 130. Additionally, content management system110 supports another client device 100B, with which the content item140A is shared. The presence of content item 140D in association withthe identification of client device 100B indicates that client device100B is also synchronizing this content item 140D with contentmanagement system 110. Thus, each client device 100 can synchronizecontent items 140 with only content management system 110 or withcontent management system 110 and other client devices 100.

Stage 1.1 illustrates the operation of a request from client device 100Ato save content item 140E to the shared content storage directory 120.However, as illustrated, the addition of content item 140E to the sharedcontent storage directory 120 would cause the total storage spaceoccupied by the content items 140 to exceed the storage allocation 130,since the size of content item 140E exceeds the remaining availablespace in the shared content directory 120 as limited by the storageallocation 130.

Stage 1.2 illustrates the operation of the selection of an unattendedcontent item 140C to be removed from the client device 100, so as tomake available sufficient storage in which content item 140E can bestored. Depending on the embodiment, either the client device 100 orcontent management system 110 determines which content items 140 toselect as being unattended. A variety of methods, discussed below can beused to determine which content items are selected as unattended. Whileonly a single content item 140C is selected in this example, in practiceany number of content items 140 may be selected, depending on the amountof storage capacity that needs to be made available.

Stage 1.3 illustrates the operation of removing the selected contentitem 140C from the client device 100A. In place of each removed contentitem, the client device 100A creates a shadow item 160C that representsthe removed content item 140C, and stores the shadow item in the samelocation in the shared content storage directory 120 as the removedcontent item 140C. Alternatively, the content management system 110 maycreate the shadow item 160C and then download the shadow file 160C tothe content storage directory 120. The shadow item includes attributesthat represent the removed content item 140C, such as the content name,path information, content attributes, and content size, but withoutcontaining the actual data of the content item 140C. By not includingthe actual data of their corresponding content items, shadow itemsrequire considerably less storage. For example, a shadow item typicallyrequires no more than the smallest file size allocation provided by theoperating system, such as 4 KB. This small size is illustrated visuallyin FIG. 1 using a vertical line, showing that the size of the shadowitem is negligible when compared to the content item 140C itself. Forexample, while the removed content item 140C may be many megabytes oreven gigabytes in size (very common for audio or video files), thestorage required for a shadow item representing such a content itemwould still be only 4 KB or so. As a result, the client device 100 isable to reduce the amount of local storage used for shared content itemsto an amount below the storage allocation 130, and thereby makeavailable sufficient space to store the newly created (or updated suchthat the new version of the content item is larger) content item 140E.Information identifying the selected (and removed) content items ismaintained on the client device 100A, to allow these items to beselectively retrieved at a later time. This information is storedlocally in client device 100 in a list 150 of stored content items thatare remotely stored in remote content item table 366 (as furtherdescribed below, not illustrated in FIG. 1A) in content managementsystem 110.

Stage 1.4 illustrates the operation of saving content item 140E to theclient device 100A once sufficient space has been made available in theshared content storage directory 120. Once the client device 100Asuccessfully saves the content item 140E to the shared storage directory120, synchronization with content management system 110 is initiated andcontent item 140E is uploaded to content management system 110. Contentmanagement system 110 still maintains full copies of all content items(including shadow items) on client device 100A.

Referring now to FIG. 1B, client device 100A and content managementsystem 110 are shown after content item 140E has been synchronizedbetween the client device 100A to the client management system 110.

Stage 1.5 illustrates the operation of client device 100A requestingaccess to content item 140C (e.g., open content item 140C using a wordprocessor, or show the content item in a file browser), wherein clientdevice 100 determines that the requested content item is represented bya shadow item. If the content item is stored locally, it is provided tothe requesting application on the client device 100A. In this case therequested content item has been removed from the client device 100A andis only stored remotely on content management system 110, so the clientdevice 100 requests content management system 110 to download therequested content item. If there is sufficient space on the sharedcontent storage directory 120, content management system 110 downloadsthe requested content item to the client device 100A; the client thenreplaces the shadow item 160C that represented content item 140C withcontent item 140C itself, which allows any requesting application toaccess the content item transparently. However, in this case, theaddition of content item 140C to the shared content storage directory120 would exceed the storage allocation 130, as depicted by content item140C extending outside the boundaries of the shared content storagedirectory 120.

Stage 1.6 illustrates the operation of selecting unattended contentitem(s) for removal from the client device 100A. In this case, theunattended content item selected is content item 140A.

Stage 1.7 illustrates the operation of removing content item 140A andreplacing it with its shadow item 160A. This removal creates enoughspace in shared content storage directory 120 for content item 140C tobe downloaded from content management system 110 and appended to itsshadow item representation without exceeding the storage allocation 130.The removed content item 140A is included in the list 150 of remotelystored content items, and content item 140C is removed from this list150, since it has been restored to the shared content directory 120.

Stage 1.8 illustrates that once content item 140C is resident on clientdevice 100A it can be opened by the requesting application. Once theprocesses illustrated by FIGS. 1A and 1B on client device 100A have beencompleted, normal synchronization can occur between client device 100Aand content management system 110 such that all changes to content items140 on client device 100A are mirrored on content management system 110.All content items 140 (even if represented by shadow items) aremaintained on content management system 110 until they are deleted fromthe shared content storage directory 120.

Overview of System Architecture

FIG. 2 illustrates the system architecture of a constrainedsynchronization system. Details about each component will be furtherdescribed in a later section, however some elements are introduced hereto provide context for the explanation of constrained synchronization.Further, as is apparent to those of skill in the art, the operations andmethods used in constrained synchronization necessarily require acomputer, and are not performed in any embodiment by mental steps by ahuman operator. Further, while the operations may make use of thefacilitates of a computer to store and retrieve information, transmitand send information, or process information, those of skill in the artappreciate that such operations are not simply generic computeroperations since they are herein performed in specific manners onspecifically defined data using the algorithms described herein, andthus require configuration of a computer in a manner different from howsuch computers are provisioned natively by their operating system andstandard applications alone. Additionally, the required configurationenhances the storage capacity of the computer, through the stepsdetailed below, over generic, general purposes computers configured withconventional operating systems and file management systems.

Client devices 100 communicate with content management system 110through a network, not shown, which can be any suitable communicationmeans providing internetworking between client devices 100 locatedremotely from content management system 110; e.g., a LAN, WAN, or WAN.In general, client device 100A with a client application 200A installedprovides content items to content management system 110. The clientapplication 200A contains the programs and protocols necessary forclient device 100A to perform the functions associated with storageconstrained synchronization. Therefore, client device 100A oftenperforms actions requested by the client application 200A. Howeverbecause client device 100A and client application 200A act together, forease of description some of these actions are referred to using “clientdevice 100A” as the operative element. The user of client device 100Ahas designated certain of the content items to be shared with clientdevice 100B, which for example, can be another computer managed by thesame user, or a computer operated by a different user. Contentmanagement system 110 notifies client device 100B and synchronizes thedesignated content items received from client device 100A with localcontent stored at client device 100B.

Content management system 110 associates each content item with anamespace corresponding to a set of content items. A namespacedesignates a directory (or “folder”) in a directory structure into whichthe given content items are stored. The association of content itemswith particular namespaces is stored in a namespace table 222. Contentmanagement system 110 associates each client with the namespaces (andcontent items therein) to which it has access, along with anidentification of the specific rights to access, modify, and delete thecontent items in each namespace. When clients 100 are synchronized to anamespace, the clients store a local copy of content items associatedwith the namespace and organize the content items according to contentlocation, if available. A user may be associated with an individualclient device 100 or with multiple clients 100; for example, a user mayhave a home computer, a work computer, a portable computer, asmartphone, and tablet computer all synchronized together. To sharecontent items, a user designates a namespace to be shared with otherusers and/or clients. Content management system 110 then synchronizesthe content items in the shared namespace(s) across the clients 100associated with the shared namespace. The content items stored atcontent management system 110 can include any type of content item,including documents, data, movies, applications, code, images, music,and so forth. The content item may also be a folder or other mechanismof grouping content items together, such as a collection, playlist,album, file archive, and so forth.

Each user is associated with an account on content management system 110that includes information specifying an amount of storage to be used forstoring content items on content management system 110. A client devicealso has a designated amount of local storage for storing thesynchronized content items, which is the size of the shared contentstorage directory 120; this designated amount is the storage allocationparameter 130 described above. For example a user's account may specifythat the user has 50 GB of storage available on content managementsystem 110, but has a storage allocation on the client device 100 ofonly 10 GB. In circumstances such as this, when the user modifies ashared content item that is stored locally, the content item mayincrease in size, and thereby exceed the storage allocation on theclient device 100. Similarly, the user may exceed the storage allocationon the client device 100 by creating and storing in the shared contentdirectory 120 a new content item to be shared and synchronized withcontent management system 110. In these cases, the amount of sharedcontent items exceeds the storage allocation for the client device 100,in which event the client device 100 is storage constrained and can nolonger maintain a local copy of all content items synchronized bycontent management system 110.

Either the client device 100 or content management system 110 isconfigured to select one or more content items to remove from the localstorage while still maintaining them remotely on content managementsystem 110, so that they can be subsequently retrieved and restored tothe client device 100. Generally, the content items that are selectedare those that are least recently accessed, either on the particularclient device 100 on which the request to access the content item ismade, or across all client devices 100 on which the content items areshared; other methods of selections are discussed further in a followingsection. In a client-based embodiment, the client application 200maintains information identifying the latest access for each sharedcontent item stored on the client device 100. When storage isconstrained, the client application 200 selects one or more of thecontent items that have been least recently accessed (herein, “LRA”). Ina host-based embodiment, content management system 110 maintains theaccess data for every content item; the system 110 updates thisinformation anytime a content item is accessed on any client device 100with which the content item is shared. LRA selection is only one of anumber of possible unattended content item selection methods (herein“UCSM”) each of which can be implemented as either a host-based orclient-based system. Any UCSM may consult the vnode reference for theeach content item to determine whether it is eligible for removal. Thevnode for each content item contains information regarding a number ofaccesses to the content item as well as other content item statusindicators including whether or not the content item is currently in useor open.

For succinctness, whenever content items are selected for removal fromresidency on a client device 100 in response to a storage constraint,the operation is referred to herein as “selecting the unattended contentitems,” since most of the UCSM operate to identify those content itemsthat are least likely to be accessed by the user. Unattended contentitem refers to content items selected by any UCSM outlined in thefollowing discussion.

Basic LRA Selection: To perform basic LRA selection, the clientapplication 200 maintains a queue of content items ordered by latestlocal access date with the least recently accessed content item at thetop of the queue. The latest access date and time for each content itemis maintained in a content access history table. An access to a contentitem includes the actions of creating, opening, previewing, or modifyinga content item. Any number of these actions can be deemed an access, forexample, an embodiment might deem an access to be either opening,modifying, or saving a content item but previewing a content item maynot be deemed an access. A cumulative sum (e.g., running total) of thestorage size is calculated for each content item listed in the queuestarting with the least recently accessed content item identified in thequeue (i.e., the content item at the top of the queue), and ending withthe content item at the end of queue. When storage is constrained, theclient application 200 determines an amount of storage space required tostore a content item, and so progresses through the queue to identifythe index of the content item for which the cumulative storage sizeexceeds the storage space requirement. The identified index is used toselect all content items above and including that index in the queue forremoval from the shared content storage directory 120 on the clientdevice 100.

These processes are further explained in Table 1. In this example, 75 MBof storage are required to store a content item. Because content items Aand B only total 70 MB, removal of these two content items does notprovide a sufficient amount of storage for the item. Accordingly,content items A, B, and C, which have a total cumulative side of 150 MBare selected (as indicated by the designation in the rightmost column),at corresponding indices 00, 01, and 02.

TABLE 1 Item Item Cumulative Index Name Local Access Time Size sizeSelected? 00 Item A 1/3/2014 10 MB 10 MB Yes 4:33 PM 01 Item B 3/24/201450 MB 60 MB Yes 5:12 PM 02 Item C 3/24/2014 20 MB 80 MB Yes 6:18 PM 03Item D 3/30/2014 80 MB 160 MB No 6:22 PM 04 Item E 5/18/2014 20 MB 180MB No 7:53 AM

Remote LRA Selection: LRA selection can also be based on remote accessesby other users that have access to the content items through contentmanagement system 110, either directly thereon, or on client devices 100which have shared versions of the content items. To accomplish this, inone embodiment, each client device 100 synchronizes its own contentaccess history table with content management system 110, for example,during normal content item synchronization operations, or at othertimes. This embodiment enables each client device 100 to maintaincurrent access information for every content item that it shares withany other client device. Alternatively, for a host based embodiment,content management system 110 may maintain a content access historytable that contains the access history for each content item across allclient devices that are designated for synchronization and sharing, sothat it has a currently updated list to use for LRA selection. RemoteLRA selection then includes the content management system 110 selectingthe least recently accessed content items whose cumulative storage sizeexceeds the required storage space. In this embodiment, this queue isordered by latest access times from all client devices that aresynchronized with respect to the content item.

Table 2 is an example of how remote LRA may be implemented. In thisexample, Content items B and C were last accessed remotely on adifferent client device on May 24, 2014 and Apr. 5, 2014 respectivelybut were both last accessed locally on Mar. 24, 2014 (as listed in Table1). This change in latest access date for Items B and C, due to theirremote accesses, moves them farther down in the queue compared to whenbasic LRA selection is used. As a result, in this example, Items A and Dare selected instead of A, B and C.

TABLE 2 Last Access Time Item Across all Item Cumulative Index NameSharing Clients Size Sum Selected? 00 Item A 1/3/2014 10 MB 10 MB Yes4:33 PM 01 Item D 3/30/2014 80 MB 90 MB Yes 6:22 PM 02 Item C 4/5/201420 MB 110 MB No 5:57 PM 03 Item E 5/18/2014 20 MB 130 MB No 7:53 AM 04Item B 5/24/2014 50 MB 180 MB No 5:12 PM

Content item Size Selection: Another factor that may be used to selectcontent items for removal is their size. In one embodiment, size is usedto minimize the number of content items that are removed and storedremotely from the client device 100. This can be accomplished byordering the queue by size (smallest to largest) instead of by accessdate. Then the required storage space value could be compared to theindividual sizes until a content item having a size that exceeds therequired storage space is identified. The client application 200 wouldthen select this content item for removal. If no single content item islarger than the required storage space then the largest content itemwould be selected and its size subtracted from the required storagespace value and the process would be repeated from the beginning of thequeue.

Table 3 is an example of this selection method. For this example, 40 MBof storage are required to store a content item. Item B is the firstcontent item by queue index that exceeds the required storage value of40 MB and so it is selected for removal from client 100.

TABLE 3 Item Item Index Name Access Time Size Selected? 00 Item A1/3/2014 10 MB No 4:33 PM 01 Item C 3/24/2014 20 MB No 6:18 PM 02 Item E5/18/2014 20 MB No 7:53 AM 03 Item B 3/24/2014 50 MB Yes 5:12 PM 04 ItemD 3/30/2014 80 MB No 6:22 PM

Content item Size and Access Time Based Selection: The size selectionmethod just described may sometimes select frequently accessed contentitems for removal. By taking both size and access time into account thecontent management system can avoid removing content items from a clientdevice 100 that may be requested by the user in the near future. In oneembodiment, this dual-variable selection method is accomplished bycalculating a weighted score for each content item, based on the amountof storage each content item contributes to reaching the requiredstorage allocation and its last access date. For example

Score=w ₁ S+w ₂ A

where S is a metric representing the content item size, A is a metricrepresenting the time since the last access to the content item, and w₁and w₂ are the weights. The weights for A and S can based on theirrelative importance, as determined by the user, by a systemadministrator, or the based on historical content item access patternsfor content items on the particular client device 100. The queue is thenbe ordered by Score and the first content item in the queue is selectedfor removal.

An example implementation of this selection method is illustrated inTable 4 below. For the purposes of this simple example, the access timemetric A is the ratio of the difference between the current time and thelatest access for the particular content item and the difference betweenthe present time and least recently accessed item access (in this casethe date used was Sep. 3, 2014). In this example, size metric is therelationship:

For s≧r: S=r/s

For s<r: S=s ² /r ²

where s is the content item size, r is the required storage space, and Sis the size metric. This piecewise function has a maximum of 1 when s=r.

In the example displayed in Table 4, the required storage space is 40 MBand the weights w₁ and w₂ are both 1. The size metric and the accesstime metric are calculated and then used to calculate the total Score,for each content item. In this example, Item B has the highest score andso is selected for removal from the client device 100. If the contentitem selected has a size smaller than the required storage space, a newrequired storage space is calculated as the difference between the oldrequired storage space and the size of the first selected content item,the score is recalculated and a new queue is generated for all contentitems using the newly calculated required storage space, and theselection process is repeated.

TABLE 4 Access Item Item Time Item Size Total Index Name Access TimeScore Size Score Score Selected? 00 Item B 3/24/2014 0.67 50 MB 0.8 1.47Yes 5:12 PM 01 Item D 3/30/2014 0.65 80 MB 0.5 1.15 No 6:22 PM 02 Item A1/3/2014 1 10 MB 0.06 1.06 No 4:33 PM 03 Item C 3/24/2014 0.67 20 MB0.25 0.92 No 6:18 PM 04 Item E 5/18/2014 0.44 20 MB 0.25 0.69 No 7:53 AM

Access Frequency and Recency Selection: In order to better selectunattended content items, other factors such as frequency can beconsidered in addition to access time. High frequency-low recencycontent items are content items that have been frequently selectedsometime in the past (e.g., more than 6 months ago) but not recently;low frequency-low recency content items are content items that havenever been frequently accessed. Frequency of access can be measuredrelative to an average frequency on a particular client device, oracross any population of client devices, or by type, name space, sourcedomain, or other content item attributes. For example, if a content itemhas not been accessed on a client device in the last four months but hadbeen accessed 25 times before that time it is likely to be more relevantto the user than a content item of similar recency that was accessedonly once in the past.

In one embodiment, the number of accesses for each content item ismaintained (at either client device 100 or content management system110) in addition to the latest access to each content item. A score isdetermined for each content item as a weighted combination of metricsrepresenting each variable. For example, a weighted score for eachcontent item, based on a metric for access frequency of a content itemand its last access date. For example

Score=w ₁ F+w ₂ A

where F is a metric representing the access frequency, A is a metricrepresenting the time since the last access to the content item, and w₁and w₂ are the weights. The weights for A and F can based on theirrelative importance, as determined by the user, by a systemadministrator, or the based on historical content item access patternsfor content items on the particular client device 100. The queue is thenbe ordered by score. A cumulative sum is calculated at each index and iscompared to the required storage space. When the required storage spaceis exceeded by the cumulative sum the index and all content items abovethat index in the queue are selected for removal from the client device100.

Table 5 illustrates one example of this selection method. In thisexample, the required storage space is 40 MB and the weights w₁ and w₂are both 1. The queue is ordered by the total score and the cumulativesum is compared to the required storage space. This results in the ItemC and E being selected for removal from client device 100.

TABLE 5 Access Access Item Time Access Number Total Item Cum. Idx NameAccess Time Score Count Score Score Size Sum Selected? 00 Item C3/24/2014 0.67 50 0.83 1.50 20 MB 20 MB Yes 6:18 PM 01 Item E 5/18/20140.44 60 1 1.44 80 MB 100 MB Yes 7:53 AM 02 Item A 1/3/2014 1 14 0.231.23 10 MB 110 MB No 4:33 PM 03 Item D 3/30/2014 0.65 32 0.53 1.18 20 MB130 MB No 6:22 PM 04 Item B 3/24/2014 0.67 26 0.43 1.10 50 MB 180 MB No5:12 PM

Any of the above UCSM may consider entire folders within a single queueindex instead of only individual files. For example, if the LRA UCSM isbeing used and a folder contains a plurality of files, where the mostrecently accessed file within the folder has an earlier access date thanall other content items in the shared content directory it may be moreefficient to select the entire folder as unattended (especially ifsignificant storage space is required). Alternatively, the combinedmetric for the folder, could be an average, median, or other statisticthat generalizes the content items within the folder allowing it to beplaced into the queue.

In the following description, any of the foregoing methods may be usedto select unattended content items for removal from a client device 100.This process of selecting unattended content items enables the enhancedstorage capabilities on the client device, as provided by theconstrained content management system 100.

Overview of Content Management System

The method of synchronization using content management system 110between client devices 100A and 100B can be explained with reference tothe architecture illustrated by FIG. 2. The following describes one of anumber of possible methods of synchronization that may be used withstorage constrained synchronization.

Content management system 110 stores content items in data store 218.Content items are stored in fixed size portions termed a block. The sizeof a block varies according to the implementation, and in oneembodiment, the blocks are 4 megabytes in size. Thus, a small contentitem is stored as a single block, while a large content item may besplit up into dozens, hundreds, or more blocks for storage at contentmanagement system 110. The metadata includes a blocklist that definesthe blocks in the content item and an ordering of the blocks in thecontent item.

Pending block table 220 maintains a list of pending blocks expected tobe received at content management system. Pending block table 220 storesan association between blocks (identified by block identifiers) and thenamespaces to which the blocks belong that clients 100 indicate will betransmitted.

Namespace table 222 stores data associating individual content itemswith namespaces and maintains data associating each namespace withclients.

Metadata server 212 is responsible for managing a request from theclient to add (“commit”) a new content item to content management system110. Metadata server 212 also receives requests to synchronize contentitems from client device 100. Metadata server 212 maintains a record ofthe last time that client device 100 synchronized with contentmanagement system 110. When a request is received from client device 100to synchronize, metadata server 212 determines any content items thathave been committed to namespaces synchronized to that client device 100since the last synchronization time stamp. In addition, metadata server212 determines any pending blocks that have been received since the lastsynchronization time stamp.

Notification server 216 is responsible for communicating with clients100, and particularly with notifying clients that new data is available.The notification server 216 maintains a list of clients 110 associatedwith each namespace at namespace table 222. When the notification server216 receives an alert from block server 214 or metadata server 212 thata new block is available for a given namespace, notification server 216identifies clients associated with the namespace from namespace table212. Notification server 216 notifies client(s) 100 associated with thenamespace to wake client(s) 100 and indicates that new blocks areavailable for the identified namespace.

A typical synchronization between two clients 100, client device 100Aand client device 100B occurs as follows. First, client device 100A addsan additional content item to the shared data. The additional contentitem is then transmitted to content management system 110. Contentmanagement system 110 notifies client device 100B that the additionalcontent item is in the shared data, and client device 100B retrieves theadditional content item from content management system 110 as clientdevice 100B. Content management system 110 maintains a list of contentitems and pending blocks that are expected to be received at contentmanagement system 110 using a pending block table 220, and notifiesclient device 100B to download blocks corresponding to a content item asblocks are received by content management system 110. Pending blocks arethose blocks that correspond to a content item that content managementsystem 110 expects to receive and are used to identify blocks that maybe provided to receiving client device 100B prior to a content itembeing committed to content management system 110.

To manage in-transit content items, content management system 110retains a list of pending blocks along with the namespace associatedwith the pending blocks. When a pending block is received, clientsassociated with the namespace are notified and can initiate a transferfor the received block. Thus, uploading clients (providing a new contentitem) and downloading clients (receiving the new content item) mayasynchronously transfer blocks to content management system 110.

Overview of Client Device

Each client device 100 is a computing device, such as a desktop, laptop,tablet, mobile device, or other system that maintains a local copy ofshared data synchronized with content management system 110 and withother clients using the installed client application 200. The shareddata may be synchronized only with clients associated with a singleuser, or may be synchronized to clients associated with multiple users.Client device 100 includes modules and applications for manipulating andadding data to the shared data, as further described with respect toFIG. 3.

FIG. 3 shows modules of client application 200. Client application 200includes various modules and data stores for synchronizing data withcontent management system 110. Client application 200 includes contentsynchronization module 310, hashing module 320, download module 330,upload module 340, and storage management module 350. Additionally, theclient application 200 maintains data stores including a file journal360, a resident file table 362, shared data 364, a remote file table366, a configuration file 368, and a block cache 370. In addition toclient application 200, FIG. 3 also indicates the storage kernelextension 384 present on the operating system of the client device. Theconfiguration of client application 200 and its associated kernelextension using these modules instantiates client application 200 as aparticular computer able to perform the functions described herein,which enables the described improvements in the storage capacity andfunctional performance of the client device.

Shared data 364 are data that has been synchronized with contentmanagement system 110, and includes content items received from contentmanagement system 110. When users add, modify, or delete content itemsin shared data 364, those changes are synchronized with contentmanagement system 110. The hashing module 320 and the block cache 370work to identify blocks that comprise content items being uploaded tocontent management system 110. The hashing module assigns a blockidentifier by performing any suitable hashing algorithm, such as MD5 orSHA-1. Content synchronization module 310 then uses these identifiers tocompare the resident blocks located in the block cache 370 with theblocks maintained by content management system 110. These modules arepresent in the current embodiment but this block implementation is notrequired for the invention of storage constrained synchronization.

When data is modified or added to the shared data 364 on the clientdevice 100 within the client application 200, the modifications to theshared data 364 are transmitted to content management system 110. Clientdevice 100 is also configured to receive notifications from contentmanagement system 110. When the client device 100 receives anotification, client device 100 queries content management system 110for modifications to shared data 364. When the shared data is modified,the client device 100 requests the modifications from content managementsystem 110 to store shared data on client device 100. In some cases, themodified data may be associated with a content item represented by ashadow item. In this case, the client device 100 may withhold therequest for modified data from content management system 110 untilaccess to the content item represented by the shadow item is requestedby an application on the client device 100. Alternatively, when a sharedcontent item is modified by another client device 100, contentmanagement system 110 may request that the constrained client device 100restore the content item represented by a shadow item such that themodification can be synchronized at the expense of other content itemsresident on the constrained client.

Within the client application 200, the file journal 360 stores a tablelisting metadata for all content items accessible to the account usingthe client application 200. Metadata includes revision date and time,namespace, and blocklists corresponding to each content item. Contentitems that are not resident or not synchronized are still included inthe file journal 360.

The resident file table 362 stores a list of files that are always keptresident on the client device 100, without regard to storageconstraints.

The remote file table 366 stores a list of files that have been selectedto be deleted from the client device and replaced with shadow items.These files are only maintained by content management system 110 andpossibly other users with access to the file.

The configuration file 368 is a file maintained by the clientapplication 200 and contains the storage allocation 120 for the clientdevice. In some embodiments the storage allocation 120 can be created bythe user or computer systems that may have control over the clientapplication 200. For example an operating system may change the storageallocation 120 so that it can maintain a sufficient amount of storagefor use by other applications.

The storage kernel extension 384 is configured to monitor requests fromapplications to the operating system 380 for access to content items,and determine whether or not the requested content items are shadowitems, and is one means for performing this function. The storage kernelextension 384 constitutes a direct modification to the structure andfunction of the operating system that enable the increase in theeffective storage capacity on the client device.

The kernel extension 384 monitors requests made to open content itemsmanaged by the client application 200. The kernel extension 384determines when requests are made to open content items managed by theclient application 200 by monitoring the file system 382 on theoperating system 380. When a request for a content item is made withinthe file system 382, the kernel extension 384 examines the pathname ofthe content item to determine whether it is within the content itemsstored within the shared content storage directory 120.

The kernel extension 384 determines whether the requested content itemis a shadow item by determining whether its size is under a thresholdsize. Alternatively, identification of a shadow item can be completedbased upon extended file attributes for content items managed by theclient application 200. A file attribute indicating a shadow item couldbe assigned to shadow items such that the kernel extension couldidentify a shadow item without examining the requested content item'ssize. If the file is determined to be a shadow item by the kernelextension 384, the kernel extension communicates the identificationinformation to the client application 200.

FIG. 4 is an interaction diagram showing one embodiment of a process foraccessing a content item not resident on the client device 100 butincluded in the file system as if the content item was resident on theclient device 100. The file system 382 receives 400 a request to open acontent item within a synchronized folder on the client device 100. Therequest may come from any application, such as a file explorer, wordprocessor, document reader, image editor, or the like. The storagekernel extension 384 intercepts 402 such file system requests, andobtains the pathname of the requested content item. The storage kernelextension 384 uses the pathname to determine 404 whether the contentitem is a shadow item. The storage kernel extension 384 may do this bychecking the size of the requested content item to determine if it isbelow a predetermined threshold, or otherwise consistent with the sizeof shadow item (4 KB). Alternatively, the storage kernel extension 384can read a file attribute extension that stores a value indicatingwhether content item is a shadow item or a regular content item. If thecontent item is not a shadow item, then the storage kernel extension 384allows the request to continue as normal and gives the file handle tothe file system so that the content item can be opened.

Upon determining that the content item is a shadow item, the storagekernel extension 384 sends 406 the request identification number(information about the request including the request type) and the filepath to the storage management module 350, passing in the file name. Thestorage management system 350 removes 408 the file name from the remotefile table 366. The storage management system 350 then wakes 412 thedownload thread, which checks content items that require synchronizationfrom content management system 110. As the requested content item hasbeen removed from the remote file table 408, the download thread can nowrequest 414 content item information from content management system 110,including the size of the requested content item in preparation for thedownload. The storage management module 350 receives 416 the sizeinformation from content management system 110, and determines 418whether storing the content item on the client device 100 will cause thepredetermined storage limit to be exceeded. If the storage limit will beexceeded by the addition of the requested content item, the storagemanagement module 350 selects 422 one or more content items stored onthe client device 100 for removal. However, if the storage limit willnot be exceeded, the storage management module 350 proceeds to download430 the content item.

In the case that the storage allocation 130 will be exceeded by theaddition of the requested content item to the shared content storagedirectory 120, the storage management module 350 selects one or morecontent items to remove, so as to make available sufficient storagespace for the requested content item before requesting a download 430thereby preventing the shared content directory from ever occupyinggreater than it's allocated space. The storage management module 350selects 422 content items for deletion by first determining 420 theunattended content items, using any of the UCSM described above. Wherethe access history of particular content items or other informationpertaining to each selection method are stored on the host system, arequest is made to the host system (not shown in FIG. 4) to update theclient application's 300 version of this information. Once the currentversion of the access history or any other required information for eachcontent item in content management system 110 has been obtained, thestorage management module 350 can determine 420 the unattended contentitems.

The storage management module then selects 422 unattended content itemsfor removal from the client device. In this embodiment, to select 422content items to remove, the storage management module 350 traverses thequeue generated by the UCSM in use to create storage space at least aslarge as the size of the requested content item to be downloaded. Theselection of the unattended content items for removal can be conductedusing any of the methods described above.

The storage management module 350 then adds 424 the names of theselected content items to the remote file table 366. Once this addition424 has been confirmed 426, the storage management module 350 removes428 the selected content items from shared content storage directory 120on client device, and then creates, for each removed content item, acorresponding shadow item that has the same metadata and location as theremoved content item, but does not contain the content information forthe content item. Shadow items may be represented in the user interfaceof the client as if they are still resident on the client device 100.FIG. 8 illustrates an example of how shadow items may be represented inthe user interface of the client device 100.

Upon removal of the selected content items, there will be sufficientstorage space on the client device 100, and the requested content itemcan be downloaded from content management system 110 without exceedingthe storage limit for the shared content storage directory 120.Accordingly, the storage management module 350 sends a download request430 to the download module 330. The download module 330 then initiates adownload 432 with content management system 110. Once the content itemis downloaded 434 to the download module 330 it is passed 436 to thestorage management module 350, which saves 438 the requested contentitem to the previously identified location and notifies 440 the storagekernel extension 384 that the download is complete. In one embodiment,the storage management module 350 appends the contents of the downloadedcontent item to the shadow item metadata, and updates the content itemattributes to indicate that content item is now no longer a shadow item.This enables the requesting application to transparently access therequested content item, using the same file handle and identificationinformation it used to initially request access to the content item. Thestorage kernel extension 384 then passes through the file handle 442 tothe file system 382, which gives the requesting application permissionto open the content item 444.

FIG. 5 is an interaction diagram showing one embodiment of a process ofsaving a content item to shared content storage directory 120 that isapproaching its storage allocation 130. The content item can be a newlycreated content item in the shared content storage directory 120, acontent item that has been relocated into the shared content storagedirectory 120, or a content item that was already in the shared contentstorage directory 120, and then modified in such a way to increase itssize. The process begins with an application making a request 500 to theoperating system's file system 382 to save a content item within thesynchronized folder. The storage kernel extension 384 monitors thisrequest and receives 502 the request-ID, file path, and size from thefile system. The storage kernel extension 384 then sends 504 thisinformation to the storage management module 350. The storage managementmodule determines 506 whether the addition of the new content item willcause the synchronized folder to exceed its storage limit. If thestorage limit will not be exceeded, the file system 382 is allowed tosave the content item as normal. In the case that the storage limit willbe exceeded, the storage management module 350 determines 508 theunattended content items and selects them for removal from the clientdevice. Once the unattended content items are selected their names areadded 512 to the remote file table 366 so that their content will not besynchronized by content management system 110. The storage managementmodule then removes the selected content items from the client device100 and replaces 514 them with shadow items, which have the samemetadata and location as the removed content items but contain nocontent. When this process is complete there is sufficient storage spacein the constrained folder for the storage management module to allow 516the original content item to be saved. The storage management modulethen wakes 518 the upload thread, which accesses 520 the metadata sothat the contents of the saved content item are uploaded 522 to contentmanagement system 110.

In addition to automatically removing content items and creating shadowitems, some embodiments also allow for the user to select particularcontent items to be stored only remotely on content management system110. This may be implemented by simply allowing the user to select froma context menu (e.g., “right-click”) on a particular synchronizedcontent item. The client application 200 would then present the userwith an option to make the selected content item remote. If the userchooses this option the content item is removed from the client device100, the name of the content item is added to the remote file table 366,and a shadow item with the same metadata and location of the originalcontent item is created to represent the original content. If the userwants to access the content item in the future the same processdescribed in FIG. 5 may be used to retrieve the content item fromcontent management system 100.

In some embodiments, the client device is configured to enable the userto select particular content items to remain resident on the clientdevice when the storage allocation 130 is reached regardless whether theUCSM in effect would otherwise select them for removal from the clientdevice 100. This embodiment offers operational improvements that allowthe user to maintain quick access to particularly important contentitems. In this embodiment, the client application 200 enables the userto access a context menu, and then select an option to force a contentitem to remain resident on the client device 100. Upon selecting, thename of the content item is added to the resident file table 362. Theresident file table 362 is subsequently accessed during the UCSM used bythe storage management module 350 shown in 422 and all content items inthe table are excluded from the selection process. For example, when agiven content item is selected for removal, the resident file table 362is examined to determine if the selected content item is listed therein;if so, the selected content item is ignored, and another content item isselected by the UCSM in effect.

Because the content associated with shadow items on a client device 100is not being synchronized it may make content management morecomplicated. For example, if a user on one client device moves a contentitem that is represented as a shadow item on a second client devicethen, if the second client device is not receiving synchronizing dataregarding the shadow item, its location may change on the first clientdevice but not on the other. For example, a content item may be deletedfrom content management system 110 completely by one client device 100while represented by a shadow item on a different client device 100. Ifthis situation occurred the user of the second client device 100 may tryto access the content item represented by shadow item only to find thatit no longer existed. To avoid these confusing circumstances, in someembodiments, the content management system 110 is configured tosynchronize shadow items for metadata only; that is, if any of theattributes of a shadow item change, content management system 110 willsynchronize the modified attributes to all client devices 100 withaccess to that content item regardless of whether the content item isrepresented as a shadow item on any of those client devices. Thus, if acontent item is deleted from one client device, the shadow itemrepresenting that content item is deleted as well on any other clientdevice 100. Alternatively in some embodiments, if a content item ismodified on another client device such that its size changes so it canfit within the remaining storage in the shared content storage directory120 on a client device 100 it may be downloaded to the client device 100even if access to the content item is not requested.

Some of the foregoing embodiments represent client-based constrainedsynchronization systems as the client application 200 is responsible forensuring that the predetermined storage allocation 130 is not exceededand for requesting data from the content management system 110. In ahost-based embodiment, illustrated in FIG. 6, content management system110 manages the constrained synchronization process, includingmaintaining information identifying the remote and resident content iteminformation for each client device 100. A host-based embodiment mayprovide the same benefits of increasing effective storage capacity on aclient device 100, while reducing the computation needed from the clientdevice, thereby improving client device 100 performance in comparison toother embodiments. The constrained content management system 600includes elements of content management system 110 shown in FIG. 2,further modified to utilize the storage management module 350 along withthe necessary data files required for the storage management module 350to function properly. Within the constrained content management systemthe metadata server 212, block server 214, notification server 216, datastore 218, pending block table 220, and namespace table 222 function inthe same manner as implemented in content management system 110.Additionally, storage management module 350 functions in a mannersimilar to when it is resident on the client device, where it isresponsible for determining when the storage space limit will beexceeded and appropriately creating shadow items. The storage managementmodule 350 is also responsible for receiving information from the clientdevice 100 about requests made by the operating system 380. When arequest is made to open one or more content items information about therequest is sent to the content management system 110 to be monitoredremotely by the storage management module 350 so that the requireddownloads are made to provide access to shadow items on the clientdevice 100. The storage management module 350 uses the clientconfiguration file 610 to provide information regarding the storageconfigurations on each client device associated with the constrainedcontent management system. The synchronization table 620 is a record ofall content items on client devices that require synchronization withthe constrained content management system 600; the content itemsincluded in this table would be a subset of the content items located inthe data store 218 since some of the content items are shadow items andrequire only metadata synchronization. Further, in this embodiment, thesynchronization table 620 may be replaced by using both a resident filetable 362 and a remote file table 366 configured such that they indicatethe client devices 100 on which each content item should be kept remoteor resident. For an embodiment using the latter configuration,implementation of metadata synchronization for shadow items is easier asthe shadow items are identified directly in the remote file table 366 ofeach client device 100. User data 630 is stored on the constrainedcontent management system 600 so that the storage management module 350can determine the unattended content items.

FIG. 7 is an interaction diagram illustrating one embodiment of aprocess of a host managed constrained storage synchronization. Anapplication on a client device requests 700 for a content item to besaved to the synchronized folders on the client device. The storagekernel extension records 702 the request ID, file path, and content itemsize and transfers 704 the information to the client application 200.The client application 200 forwards 706 the content item sizeinformation to the storage management module 350 on the constrainedcontent management system 600. The storage management module 350requests 708 the storage limitation for the particular client from whichit received 706 the content item size information from the clientconfiguration file 610. The storage management module 350 determines 712that the storage limit will be exceeded by comparing the size inaddition to the other content items resident on client device 100 to thestorage allocation received from the client configuration file 610. Thestorage management module 350 requests 714 the content data on theclient from the synchronization table 620 so that it may select contentitems to remove from the client from the synchronized content items onthe client. The synchronization table responds 716 with the synchronizedcontent data for the particular client. The storage management module350 requests 718 user access data from user data 630 stored on a hostdevice to use to determine LRA content items. Once this data is received820 from the user data table 630. The storage management module 350 candetermine 722 the LRA content items and select 724 those that should beremoved from the client to provide the required storage space. Thestorage management module 350 sends requests to remove content items andcreate shadow items 728 to the client application 200. It gives 730permission to the client application 200 to complete the originalrequest 700 to save a content item. Finally the storage managementmodule updates 732 the user data to reflect the first content itemaccess for the saved content item and then requests 734 synchronizationof the client device 100 from the metadata server 212 since a newcontent item is available for upload.

FIG. 8 illustrates an example a user interface of client device 100operating in coordination with a content management system providingconstrained synchronization. A synchronized files folder 800 serves asthe shared content storage directory 120. Folder 800 contains a numberof content items, each represented by a corresponded icon 810A, a .m4amusic file, 810B, a .xlsx spreadsheet, 810C, a .docx word processingfile, 810D, a .mat Matlab file, and 810E, a .jpg image file. Each icon810 is overlaid with a status icon 820 that indicates the storage statusof the content item.

Status icon 820A (“check icon”) indicates that the content item iscurrently resident on the client device 100 and is synchronized with thecurrent version of the content item maintained by content managementsystem 110.

Status icon 820B indicates that the content item will be resident on theclient device 100 once synchronization with content management system110 is complete.

Status icon 820C indicates that the content item is a shadow item and isnot currently resident on the client device but still maintained oncontent management system 110.

Status icon 820D indicates that the content item is resident on theclient device and synchronized with its version maintained by contentmanagement system 110. Additionally, the green circle with the pin icon840 indicates that the content item has been chosen to remain residenton the client device 800 during a storage constraint.

FIG. 9 is a concept diagram illustrating an alternate embodiment ofconstrained synchronization, which predicts user access to particularcontent items remote to a client device, and downloads the predictedcontent items in advance of the access. This approach offers a furtherimprovement in the operation of the client device by eliminating in mostcases the time a user may have to wait to retrieve the content item overthe network from content management system 110. A retention score 900 iscalculated for each content item 140 within a shared content storagedirectory 120. This score is a measure of the predicted importance of acontent item and can be calculated as a function of latest access time,or a number of other factors determined to be predictive of a userrequest, as explained in a later section. Additionally, each contentstorage directory 120 is configured with a retention score threshold910, which may be specified by the user or set at a predetermined value.Whenever the predicted importance of a content item, as measured by theretention score 900 of the same content item, exceeds the retentionscore threshold 910 of a particular shared content storage directory 120on a client device 100 with access to the content item, the content itemis downloaded to the shared content storage directory when it is remoteto the client device and maintained within the shared content directoryif it is resident on the client device.

Stage 9.1 illustrates a typical state of a content management system,which predicts user access to content items. In this illustration,content management system 110 manages two client devices 100A and 100B.Shared content storage directories 120A and 120B are located withintheir respective client devices. Shared content storage directory 120Astores content items 140A, 140B, and 140C while shared content storagedirectory 120B stores content item 140D and a shadow item representation160A of content item 140A. Synchronized versions of all content items140 are stored on content management system 110.

Additionally, each content item 140 has a corresponding retention score900, where 900A is the retention score for content item 140A, 900B isthe retention score for content item 140B and so forth. Each sharedcontent storage directory is also configured with a retention scorethreshold 910, where 910A is the retention score threshold for sharedcontent storage directory 120A and 910B is the retention score thresholdfor shared content storage directory 120B.

In stage 9.1, content item 140A is not maintained in shared contentstorage directory 120B. Though in this case there are no content itemsresident within a shared content storage directory 120 that have aretention score 900 lower than the retention score threshold 910, thisscenario is possible if traits from other embodiments, describedpreviously or in a following section, are used in addition to those fromthis embodiment. For example a storage allocation may still be in affectand so if the storage allocation is sufficiently large it may not benecessary to keep a file remote even if it has a retention score 900lower than the retention score threshold 910.

In stage 9.2, a user of client device 100A performs a user action 920 oncontent item 140A that is considered an access to content item 140A.Because, in this example, retention scores 900 are calculated as afunction of latest access time, the retention score 900A of content item140A increases from 20 to 60 (The magnitude of this change is arbitraryfor the purpose of this example. Details on retention score calculationare provided later and may not result in the same score change).

In stage 9.3, the content management system 110 or, in some embodiments,the client application on client 100B, determines that the retentionscore 900A of content item 140A is greater than or equal to theretention score threshold 910B of the shared content storage directory120B where content item 140A is remote. Because the retention score 900Aexceeds the retention score threshold 910B, the content item 140A isdownloaded to client device 100B and stored in shared content storagedirectory 120B.

Similar to the UCSMs there are a number of retention score calculationmethods. Generally retention scores can be normalized against userbehavioral attributes, resulting in retention scores for the samecontent item that are different for each client device, or global sothat scores are the same for each client device. The advantage ofnormalized retention scores is that they level out differences in userbehavior. For example, if the retention score is a function of thelatest access time of a content item where the score increases as thetime between the present time and the latest access time decreases, amore active user would drive up the retention scores of content itemsshared with that user when compared to content items shared with a lessactive user. If the retention scores are not normalized for a thirduser, sharing with both the active user and the less active user, theretention scores would lose their predictive quality as only the itemsfrom the active user would have the highest retention scores even thougha recent access by the active user is less predictive of an access bythe third user than is a recent access by the less active user. Whenevera retention score is normalized it can be normalized to an attribute ofa particular user or a particular content item.

The following methods are examples of methods for determining aretention score, or a score anticipating a user access of a contentitem. Additionally, a retention score may use a combination of thefollowing methods to create the most predictive measure of predictedimportance. Typically the retention score increases as the predictedimportance of a content item increases, however the opposite can be trueif convenient for an embodiment. In this case the correspondingretention score threshold would be a minimum value where if theretention score of content item was less than or equal to the retentionscore threshold it would be downloaded to the corresponding sharedcontent storage directory. For the purposes of this discussion thedefault case of an increasing retention score will be assumed.

Latest Access Scoring: For latest access scoring the retention score ofa content item is a function of the latest access time of that contentitem. The retention score could simply be the inverse of the differencebetween the current time and the latest access time in seconds:

RS=1/(t _(C) −t _(A))

where RS is the retention score, t_(C) is the current time, and t_(A) isthe latest access time.

If normalization is needed for the particular embodiment a variety ofuser attributes may be used such as a user's or client device's accessfrequency defined as the number of accesses of any shared content itemby a particular user or on a particular client device within apredetermined time period. Alternatively, the average latest access timeof content items shared with a particular user or client device may beused.

Access Frequency Scoring: For access frequency scoring, the retentionscore of a content item increases with an increase in the number ofaccesses to the same content item within a predetermined time period. Tonormalize access frequency scoring the access frequency for a givencontent item could be divided or otherwise scaled by the average accessfrequency for all content items on a client device or shared with auser.

Location Related Access Scoring: For location related access scoring,the retention score of a first content item is a weighted combination ofthe latest access time, access frequency, or any other characteristic ofthe content item itself and the same characteristic of additionalcontent items stored in the same folder as the first content item. Thisimplies that accesses to content items within a folder are predictive ofaccesses to other content items within the same folder.

Similar Access Scoring: For similar access scoring, the retention scoreof a first content item is a weighted combination of the latest accesstime, access frequency of the content item itself and the samecharacteristic of additional content items with similar attributes asthe first content item. Attributes may include content item type, size,location, users with access to the content item, etc. This implies thataccesses to similar content items are predictive of future accesses to acontent item.

Criteria Based Retention Scoring: For criteria based retention scoring,the retention score of a content item is based on the number ofpreviously identified predictive criteria satisfied by the content item.For example, access to a content item by another user within 24 hours,an access frequency greater than 5 accesses in the last week, andaccesses to sufficiently similar content items within the last 3 daysmay all be criteria predetermined to be predictive of an attempt toaccess a remote content item within the next 6 hours. Therefore, theretention score of a content item may increase by a predeterminedmagnitude for each of the criteria satisfied by the content item. Themagnitude of the increase for a particular satisfied criterion may beproportional to the predictive strength of the particular criterion.

FIG. 10 illustrates a system environment for a content management systemusing predicted content item importance for constrained synchronization.Most of the modules of the constrained content management system 600that are present in FIG. 10 perform similar or identical functions tothose described with reference to FIG. 6 except where noted in theforegoing section. Therefore, the functions of all modules withincontent management system 1000 are not explained in detail in thissection.

Content management system 1000 includes metadata server 212, blockserver 214, notification server 216, data store 218, pending block table220, namespace table 222, storage management module 350, clientconfiguration file 610, synchronization table 620, user data 630,retention score table 1010, and retention score module 1020. Clientconfiguration file 610 and user data 630 have significant changes overprevious versions described in FIG. 6. Client configuration file 610 ismodified to include the retention score threshold for each sharedcontent storage directory of each client device, while user data ismodified to include user data relevant to the retention scoring methodbeing used. The retention score module 1020 takes in user data 630 anddata from the data store 218 to generate the retention score table 1010.The retention score table is a table enumerating the retention score ofeach content item managed by the content management system 1000. Aseparate retention score table may exist for each client device ifnormalization is being used to calculate retention scores. Whenever theretention score of a content item is updated, the retention score module1020 consults the client configuration file 610 and the synchronizationtable 620 to determine if the corresponding content item to the recentlychanged retention score is remote on any client devices and if itexceeds any of the retention score thresholds of those client devices.If a retention score threshold is exceeded the retention score modulerequests that the storage management module 350 perform the necessarydownload and replacement of the representing shadow item.

FIG. 11 illustrates the software architecture of the client application1100 for another embodiment of constrained synchronization. Thisembodiment conducts all downloading of remote content items, removal ofunattended content items, and creation of shadow files while the clientdevice is determined to be idle by the client application. This changein timing of the constrained synchronization process improves the clientdevice by offering a functional improvement over the previouslydescribed embodiments while providing a similar increase in effectivestorage capacity. To perform these functions the idle state triggeredembodiment modifies the system architecture illustrated in FIG. 3. Inthis embodiment, client application 1100 is comprised of contentsynchronization module 310, retention state module 1110, file journal360, resident file table 362, shared data 364, remote file table 366,configuration file 368, and block cache 370. The content synchronizationmodule 310 is further comprised of hashing module 320, download module330, upload module 340, and storage management module 350. The retentionstate module 1110 is further comprised of state calculation module 1120,state comparison module 1130, action module 1140, and system statusmodule 1150. Unless otherwise specified all previously mentioned modulesand data tables have the same function as previously described slightlymodified as one skilled in the art would recognize to accommodate thenew modules. Any major modifications are explained below.

System status module 1150 uses storage kernel extension 382 to measuresystem activity on operating system 380. System activity can be measuredusing metrics for processor activity including but not limited to thenumber of non-idle processor cycles as a ratio of processor frequency oranother CPU utilization metric (with or without adjustment for multipleprocessor cores), the number of threads, or the number of processes of aclient device 100. Network activity metrics may also be used includingnetwork utilization, defined in bits per second or packets per second,as a ratio of the maximum speed for a particular port or connection.Additionally, memory usage metrics including the amount of available orfree random access memory (RAM) may be used to measure system activity.The system status module 1150 may use the activity metrics mentionedabove or any other suitable activity metrics, individually or incombination to measure overall system activity.

When the measure of system activity is below a predetermined activitythreshold, the system status module 1150 reports to the retention scoremodule 1110 that the client device is currently idle. This activitythreshold may be defined as a percentage of the total computationalresources of the client device, as defined by an activity metric, or theactivity threshold may be defined as a particular value of an activitymetric. For example, an activity threshold may be defined as the stateof the client device 100 using less than 25% of available processingresources. Alternatively, the activity threshold may be defined as thestate when the other processes of the client device 100 are, in total,using less than 2 GB of memory, or that there is at least 4 GB of totalmemory available on the client device.

When the client device 100 has been determined as being in an idle stateby the system status module 1150, state calculation module 1120determines the retention state of the shared content storage directory120. Generally the retention state consists of the content itemsresident on the client device and a set of attributes corresponding tothose content items. These attributes may include content item size,latest access time, access frequency, directory location, or any othersuitable attribute that would be indicative of a content item'simportance for retention on a client device. Additionally, the retentionstate could be represented by a set of statistics calculated using atleast one of the attributes listed above.

Comparison module 1130 receives the retention state from the statecalculation module 1120, it then compares the current retention state ofthe shared content storage directory 120 with a predetermined thresholdretention state, defined in the configuration file 368, that may bespecified by the user. The threshold retention state is a set ofcriteria pertaining to the attributes or calculated statistics of theclient device included in the retention state. The comparison module1130 determines whether the current retention state satisfies thecriteria of the threshold retention state. If these criteria areviolated (e.g. not satisfied), the comparison module 1130 reports thecontent items corresponding to the attributes, or the calculatedstatistics based on those attributes, that violate the thresholdretention state criteria to the action module 1140.

Action module 1140 receives the report from the comparison module 1130.It then determines what actions will bring the retention state back towithin the threshold retention state criteria. These actions may includeremoving content items from the shared content storage directory 120 andreplacing them with shadow items, or replacing shadow items representingremote content items with the content items themselves. Once theseactions have been determined, the action module 1140 requests thatcontent synchronization module 310 complete the required actions.

Alternatively, idle state triggered constrained synchronization could beconducted by the content management system itself further reducing thecomputation burden on the client device and increasing deviceavailability for other uses. FIG. 12 illustrates a system environmentthat completes this task. Constrained content management system 1200 iscomprised of metadata server 212, block server 214, notification server216, data store 218, pending block table 220, namespace table 222,storage management module 350, client configuration file 610,synchronization table 620, user data 630, retention state table 1210,retention state module 1220. Unless otherwise specified all previouslymentioned modules and data tables have the same function as previouslydescribed slightly modified as one skilled in the art would recognize toaccommodate the new modules. Any major modifications are explainedbelow.

In this version of the embodiment, client application 200 on a clientdevice connected to the content management system 1200 reports to thecontent management system 1200 on the status of the client device. Whenthe client device is idle the content management system 1200 uses theretention state module 1220 to determine the retention state of theshared content storage directory 120 on the idle client device. Theretention state module then updates the retention state table 1210,which contains the current retention state of all client devicesconnected to the content management system 1200. The retention statemodule 1220 then conducts steps similar to retention state module 1110using potentially similar submodules, as described during the discussionof FIG. 11.

The retention state of a shared content storage directory can bedetermined using a variety of methods. Generally, the retention state iscriteria based and is maintained periodically whenever the clientapplication determines that the client device is idle. However, it isalso possible to implement the retention state and threshold retentionstate numerically such that each state is represented by a statisticcalculated using the attributes of the content items resident on theclient device. If the retention state is criteria based, the thresholdretention state is a set of criteria that the content items within theshared content storage directory must satisfy. Additionally, in the caseof a criteria based retention state, the user may be given an option tochoose the retention state criteria thereby allowing customization ofthe categories of content items resident on a client device 100.

The period used to check each client device can be a predetermined valueof the content management system, set by the user, or determined basedon usage patterns of the particular client device. For example, if auser accesses content items on their client device on average every 24hours the period could be set to ensure that the shared content storagedirectory is maintained before 24 hours passes.

As an alternative to checking a shared content directory periodically,another embodiment could maintain a shared content directory only whenthe shared content directory satisfies a second set of criteria thatindicate urgency, for example, nearing a hardware storage limit.

Storage Space Criteria: One possible set of criteria is to have astorage allocation criteria. For example, a storage allocation could beset at 20 GB but instead of behaving like the previous embodiments, thecontent management system would allow the content items stored on theshared content storage directory to exceed the criteria value (in thisexample 20 GB) until the device was idle. Then a similar process ofdetermining unattended content items could be used to remove theappropriate content items and satisfy the storage space criteria for theshared content storage directory.

Access Time Criteria: A second criterion could be an access timecriterion. For example, the criterion could state that no content itemwith a latest access time earlier than a predetermined time interval inthe past can be resident within the shared content storage directory.These content items would be allowed to remain resident within theshared content storage directory until the client device was idle. Atthat point the retention state module would simply request the removalof all content items with a latest access time earlier than thepredetermined time interval.

Content Item Size Criteria: Another set of criteria is the content itemsize criterion. For this method, a threshold on the individual contentitem's size is set. Therefore, whenever the device is idle any contentitem over or under that threshold is removed from residence on theclient device.

Access Frequency Criteria: Finally an access frequency criterion is usedto set a minimum number of accesses within a predetermined time intervalrequired to remain resident on a client device. If a particular contentitem is not accessed frequently enough it is removed from the clientdevice whenever it is idle.

Note that this list of retention criteria is not exhaustive.Additionally, these criteria may be used in conjunction with each otherresulting in more complex rules.

FIG. 13 is a flow diagram illustrating the function of idle statetriggered constrained content management. First the system checks 1300to determine whether a particular client device is idle. This step iscompleted either periodically or in response to the content storagedirectory reaching a predetermined threshold. If the device is idle, thesystem determines 1310, the retention state of the client device. Thenthe system compares the current retention state of the shared contentstorage directory to the retention state criteria for the shared contentstorage directory. If the criteria are satisfied by the currentretention state of the shared content storage directory the systemresumes checking 1300 to determine whether the client device is idle. Ifthe retention state criteria are violated the system identifies 1330actions to perform on the shared content storage directory that arerequired for the shared content storage directory to meet the retentionstate criteria. The system then performs 1340 those actions on theshared content storage directory to conform to the predeterminedretention state criteria.

The foregoing description of the embodiments of the invention has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the invention to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of theinvention in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the invention may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the invention may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the invention be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsof the invention is intended to be illustrative, but not limiting, ofthe scope of the invention, which is set forth in the following claims.

What is claimed is:
 1. A computer-implemented method of synchronizingcontent items, the method comprising: storing by a client device ashadow item, wherein the shadow item represents a content item stored ona separate host device that is remote to the client device and theshadow item is comprised of metadata of the content item stored on theclient device, the metadata including a size of the represented contentitem, without content data of the content item being on the clientdevice; determining from the metadata of the shadow item that the sizeof the represented content item exceeds a remaining amount of storageallocated on the client device; selecting at least one content item toremove from the client device, each selected content item already storedon the host device, by: accessing a list of content items synchronizedbetween the client device and the host device, the list excluding allcontent items marked to remain resident on the client device; andselecting one or more content items such that the total size of theselected content items is greater than or equal to the size of therepresented content item; replacing each selected content item with ashadow item representing the selected content item on the client device;retrieving the represented content item from the host device; andreplacing the shadow item representing the represented content item withthe represented content item.
 2. The method of claim 1, whereinreplacing each selected content item further comprises: creating ashadow item on the client device representing each selected contentitem; and deleting by the client device each selected content item fromthe client device.
 3. The method of claim 1, wherein replacing theshadow item representing the represented content item further comprises:downloading by the client device the represented content item from thehost device; and storing the downloaded content item on the clientdevice.
 4. The method of claim 1, further comprising prior to replacingeach selected content item with a shadow item, synchronizing eachcontent item with the host device.
 5. The method of claim 1, wherein anamount of storage allocated on the client device is read from aconfiguration file configured in response to settings specified by theuser.
 6. The method of claim 1, wherein replacing each selected contentitem with a shadow item representing the selected content item on theclient device further comprises: adding by the client device anidentifier for each selected content item to a list maintained by theclient device indicating content items for which the client device doesnot request synchronization from the host device.
 7. The method of claim1, wherein replacing each selected content item with a shadow itemrepresenting the selected content item on the client device furthercomprises: sending by the client device to the host device a messagespecifying an identifier for each selected content item, whereby theidentifier is added to a list of content items maintained by the hostdevice that the host device does not synchronize with the client device.8. A non-transitory computer-readable storage medium storing executablecomputer instructions that, when executed by a processor, cause theprocessor to perform steps comprising: storing by a client device ashadow item, wherein the shadow item represents a content item stored ona separate host device that is remote to the client device and theshadow item is comprised of metadata of the content item stored on theclient device, the metadata including a size of the represented contentitem, without content data of the content item being on the clientdevice; determining from the metadata of the shadow item that the sizeof the represented content item exceeds a remaining amount of storageallocated on the client device; selecting at least one content item toremove from the client device, each selected content item already storedon the host device, by: accessing a list of content items synchronizedbetween the client device and the host device, the list excluding allcontent items marked to remain resident on the client device; andselecting one or more content items such that the total size of theselected content items is greater than or equal to the size of therepresented content item; replacing each selected content item with ashadow item representing the selected content item on the client device;retrieving the represented content item from the host device; andreplacing the shadow item representing the represented content item withthe represented content item.
 9. The non-transitory computer-readablestorage medium of claim 8, wherein replacing each selected content itemfurther comprises: creating a shadow item on the client devicerepresenting each selected content item; and deleting by the clientdevice each selected content item from the client device.
 10. Thenon-transitory computer-readable storage medium of claim 8, whereinreplacing the shadow item representing the represented content itemfurther comprises: downloading by the client device the representedcontent item from the host device; and storing the downloaded contentitem on the client device.
 11. The non-transitory computer-readablestorage medium of claim 8, wherein the instructions, when executed,cause the processor to perform further steps comprising: prior toreplacing each selected content item with a shadow item, synchronizingeach content item with the host device.
 12. The non-transitorycomputer-readable storage medium of claim 8, wherein an amount ofstorage allocated on the client device is read from a configuration fileconfigured in response to settings specified by the user.
 13. Thenon-transitory computer-readable storage medium of claim 8, whereinreplacing each selected content item with a shadow item representing theselected content item on the client device further comprises: adding bythe client device an identifier for each selected content item to a listmaintained by the client device indicating content items for which theclient device does not request synchronization from the host device. 14.The non-transitory computer-readable storage medium of claim 8, whereinreplacing each selected content item with a shadow item representing theselected content item on the client device further comprises: sending bythe client device to the host device a message specifying an identifierfor each selected content item, whereby the identifier is added to alist of content items maintained by the host device that the host devicedoes not synchronize with the client device.
 15. A computer-implementedmethod of synchronizing content items, the method comprising: storing bya client device a shadow item, wherein the shadow item represents acontent item stored on a separate host device that is remote to theclient device and the shadow item is comprised of metadata of thecontent item stored on the client device, the metadata including a sizeof the represented content item, without content data of the contentitem being on the client device; determining from the metadata of theshadow item that the size of the represented content item exceeds aremaining amount of storage allocated on the client device; selecting atleast one content item to remove from the client device, each selectedcontent item already stored on the host device, by: accessing a list ofcontent items synchronized between the client device and the hostdevice, the list sorted by client device that most recently accessedeach content item; and selecting one or more content items based on theclient device that most recently accessed each content item such thatthe total size of the selected content items is greater than or equal tothe size of the represented content item; replacing each selectedcontent item with a shadow item representing the selected content itemon the client device; retrieving the represented content item from thehost device; and replacing the shadow item representing the representedcontent item with the represented content item.
 16. The method of claim15, wherein replacing each selected content item further comprises:creating a shadow item on the client device representing each selectedcontent item; and deleting by the client device each selected contentitem from the client device.
 17. The method of claim 15, whereinreplacing the shadow item representing the represented content itemfurther comprises: downloading by the client device the representedcontent item from the host device; and storing the downloaded contentitem on the client device.
 18. The method of claim 15, furthercomprising prior to replacing each selected content item with a shadowitem, synchronizing each content item with the host device.
 19. Themethod of claim 15, wherein an amount of storage allocated on the clientdevice is read from a configuration file configured in response tosettings specified by the user.
 20. The method of claim 15, whereinreplacing each selected content item with a shadow item representing theselected content item on the client device further comprises: adding bythe client device an identifier for each selected content item to a listmaintained by the client device indicating content items for which theclient device does not request synchronization from the host device.